Characteristic Features of the Anomalous Magnetic Properties of Some Mixed Terbium–Yttrium Ferrite Garnets at Low Temperatures

This article is devoted to the experimental investigation of the occurrence of the so-called “low-temperature Belov point” T
_B
predicted previously for the mixed rare earth-yttrium iron garnet compounds RE
_x
Y
_3-
x
Fe
₅
O
₁₂
possessing or not a magnetic compensation point T
_comp
. Isothermal magnetizations MT(H) of two single crystals of Tb
_x
Y
_3-x
Fe
₅
O
₁₂
with rare-earth (RE) = Tb, and x = 1.98, 0.37, were measured in the 4.2-300 K range, using dc magnetic field H up to 100 kOe applied in one or more of the three main crystallographic directions. Anomalies are observed in the region of T
_B
^TbIG
≈ 58 K, the previous estimated value for TbIG (x = 3), in the temperature dependences of the paraprocess susceptibility χ
_p
, the parameter |b| of the bH
²
term of the quadratic expansion of MT(H) curves, and other pertinent magnetic characteristics. For x = 1.98 with T
_comp
= 137 K and when H is applied along (110), the χ
_p
- and |b|-maxima take place close to T
_B
^TbIG
. The anisotropy of the χ
_p
-data gives evidence for a spontaneous noncollinear structure around the easy axis of magnetization (111) at 4.2 K. For x = 0.37 which has no T
_comp
point and when H is applied along (100), the χ
_p
- and |b|-maxima are clearly disturbed by the sign anomalies caused by the noncollinear structure instability mechanism due to the crossing of the Tb
³⁺
levels at the critical point T* = 16 K and by the lower continuous change from the easy axis <;100> to a low-symmetry angular phase (uuw) due to the first spontaneous spin reorientation phase transition at T
_SR1
= 40 K. When x decreases from 3 to 0.37, it is found that despite different anisotropy effects of the Tb
³⁺
ion, its magnetic moment m
_Tb
close to T
_B
^TbIG
is practically constant (~4.99 in μ
_B
per Tb
³⁺
ion).